Meta's Sapling source-code management system

The internal history of this, as I understood it (a couple of years out of date, and not near any team responsible - just as a user), is that FB started with mercurial. They then had to heavily customise hg to make it work at the ever greater scales they had internally with more and more code and developers working on it. Until they had effectively completely rewritten the back-end to use a Facebook specific, distributed object store - that's the "eden" bit in the source code I think (maybe simplified / pared-down for external use, I don't know). The front-end hg tools I think were heavily modified too. Sl however is a from scratch rewrite. I think it started out as a wrapper around the hg tools, but grew into a standalone front-end. There was also an effort to reimplement the back-end in Rust, along with front-end tooling for that - I think that's the "Mononoke" bit in the code, IIRC.

Last I remember, I /think/ the developer workflow still had some odd cases where you needed to use the hg commands, but for nearly all stuff you could use sl for your daily work-flow.

I presume that progressed to the stage where the completely rewritten Facebook^WMeta front-end + backend, sl / mononoke, can do everything itseflf, and is feature complete - and hence this can be released as "sapling" (retro-fitted name).